The present invention relates to a pumping method of discrete elements solid state laser systems pumped by semiconductor laser diodes, which sends a pump beam through an active element comprising a first face first crossed by said pump beam, and a second face met as second by said pump beam, a pumping axis being associated to the pump beam, the active element being inserted in a cavity to which a cavity propagation axis is associated to.
An apparatus developed according to the above method is also described by way of a non limiting example. Operation of such an apparatus can be in a continuous wave or pulsed regime through appropriate modulation of the electromagnetic field in the cavity.
According to the conventional side pumping scheme of active crystal elements in solid state laser systems, the pumping light propagates and is absorbed into the active material along the crosswise direction to the propagation direction of the laser mode; the need of a long enough crosswise path of the pump light in the active element to warrant a high pump light absorption, i.e. efficient pumping, generally involves the energetic activation of a much larger section of the active element than the cross-section of the fundamental eigenmode of the stable cavity in which it operates and a consequent operation of the laser oscillator on several higher modes, which saturate power extraction. Once the absorption efficiency requirement of the pump light has been established, the laser systems so conceived do not ensure control of the number of higher order modes being excited, and consequently of the beam quality.
On the other hand, a so-called longitudinal pumping or end pumping technique is known, which ensures concentration of the gain area in the active medium inside the volume occupied by the fundamental mode, so as to cause oscillation on the mode TEM0,0, the propagation of which occurs at the diffraction limit.
Thus, the pump energy is concentrated in a very restricted volume of active material with a length nearly equal to the absorption length of the pumping beam, with a cross section generally equalling the size of the focal spot. Through appropriate design provisions of the laser cavity, pumping optics and choice of the active material it is often possible for the volume activated by the pump energy to be included in the volume of the cavity fundamental mode TEM0,0 and cause oscillation of the laser cavity on the cavity fundamental mode, or TEM0,0, with a maximum energetic extraction efficiency as possible.
The rich literature available on the matter indicates that for limited pump powers (about <2 W), the optimal active volume for selecting TEM0,0 in active media with strong thermo-optic or thermo-mechanical effects is nearly totally contained inside the fundamental mode itself, and the ratio between the laser mode diameter and the pump beam diameter in the active crystal may exceed the unit. These diameters are evaluated as the double distance from the propagation axis to the point where the beam intensity reaches 1/e2 times the peak value. Should it be wished, vice-versa, to increase the pump power, it would be necessary to reduce the ratio between the laser mode diameter and the pump beam diameter below the unit for limiting the strong losses due to the optical aberration on the edges of the so-called “thermal lens”, which is due to the heat generated by the pump power absorbed in the active material. Thus, only the most external and energetically less significant circle of the cavity laser mode is subject to a non parabolic phase modulation and to the losses caused by repeated passes in the resonant cavity.
However, the value of the above overlaying ratio should remain next to the unit for selecting the fundamental mode TEM0,0 and avoid oscillation of higher order modes. For a pumping power ranging between 20 W and 50 W, some sources specify a ratio of about 0.83 as the optimal value for selecting the fundamental mode. Based on this value, these sources indicate differential efficiencies (the ratio between the output power change and the pumping power change when the laser operates over the threshold) over 0.4 for operation on the fundamental mode utilizing Nd:YVO4 as the active medium at 1064 nm laser wavelength.
At equivalent laser power being generated, the longitudinal pumping will be the most efficient one for producing solid state laser sources with a high beam quality, measured through the parameter M2, i.e. operating near diffraction limit, with M2 about 1, therefore featured by a high brightness. The source brightness (radiant power per surface unit per unit of solid angle) is proportional to the intensity (power per surface unit) obtainable by focusing the laser beam; therefore, it is a basic feature in the applications involving interaction with the materials, such as the ones of laser micro-machining, marking and engraving. In these material processing applications, a high source brightness will ensure:                a high interacting performance with various materials through the high irradiance that can be obtained;        a high spatial resolution of the machining details through the high beam quality;        high execution speed due to the high power available.        
The ratio between the power available and the minimum size of the focal spot indicates the brightness of the laser source, defined as the power per unit of solid angle and surface. Quite generally, a high brightness laser will be more efficient than a low brightness laser when interacting with the material.
Therefore, solid state laser sources with a very high brightness are required, which are designed for their easy reconfiguration, in order to stress the beam quality or source average power feature for the specific requirements of the application they are used for.
It is particularly important to obtain such laser sources using Neodymium doped crystalline active materials, such as Nd:YAG, Nd:YVO4, Nd:YLF, Nd:YAP, Nd:GdVO4, Nd:BYF, Nd:SFAP or Ytterbium doped ones, such as Yb:YAG, Yb:YLF, Yb:SFAP.
Providing such a source entails some critical factors, which may represent some drawbacks and limit the extraction of the average powers required in the range between 2 and 100 W or compromise the beam quality required at such operating powers.
A first more relevant drawback is due to the pumping process and laser action, which deposits residual heat in the active material. In particular, the above phenomenon of thermal lens deteriorates laser power extraction, since the most external portion of the laser mode passing through the active material undergoes phase aberrations, which cause a net reduction of power circulating in the cavity, as well as a worsening of the laser beam quality.
In addition, operating a laser apparatus with an intensive thermal lens in a giant pulse operation, i.e. a repetitive Q-switching regime, may prove difficult.